Scrap lead acid batteries typically contain metal components, for example plates, grids, terminals and metal casing components. These metal components are elemental metals or metal alloys. The battery also has a component known as a battery "paste" or "mud." Battery paste is derived from two chemically distinct sources from within a used battery. The major part of the paste is derived from the lead oxide paste (plattnerite, PbO.sub.2) originally coated on the grids during manufacture of the battery, and which takes an active part in the electro-chemical functioning of the battery. The lesser fraction of the paste consists of corrosion products, i.e., oxidation products of the grid alloy. Corrosion of the grids occurs during the life of the battery, but particularly when the battery is in a low state of charge. The grid alloy consists of lead and at least one alloying component from the group antimony, arsenic or tin. Thus, this fraction of the paste from used batteries consists of oxides and sulfates of lead and also contains one or more alloying components such as antimony in an oxidized state.
It has been practice to charge used storage batteries whole or shredded into a blast furnace together with coke and chalk. The lead oxide and sulfate are thereby reduced to metallic lead and a slag containing iron oxide, silica and calcium oxide is produced. In some cases, concentrated scrap including the paste and metallic constituents is treated in electro-furnaces, reverberatory furnaces or slowly-rotating drum furnaces with a reductant, iron and an alkali flux to bind sulphur. Such processes result in an alkali containing slag which is difficult to dispose of without contaminating the environment. The lead produced typically contains undesirable quantities of alloying elements such as antimony and is called "hard" lead.
Another practice was to first shred or comminute the batteries and separate out the rubber or plastic of the battery casing before processing. By adding water to the small particles produced by comminution, the lighter particles float to the surface and are removed. The heavier particles, including lead and lead compounds, are then passed over a rotary vacuum filter to reduce the water content to about 10%. By this means a wet battery paste was made as filter cake and dried. This dried filter cake is subjected to various processes. One process utilizes a long rotary kiln where the pre-dried battery paste is mixed with coke, soda ash and iron chips on a belt feeder before being charged into the oil-fired kiln where the mixture is heated to 1300.degree. C. to accomplish reduction of the PbO and PbSO.sub.4 to molten lead. After the PbO is reduced, oxides of arsenic, antimony and other impurities are also reduced to base metals.
Another practice is to separate batteries into fractions and to treat the paste fraction separately from the fraction consisting of metals and alloys.
Treatment of the metals fraction is relatively straightforward, since the starting material is free of sulfates. A suitable method of separating lead from antimony in the alloy is described in U.S. Pat. No. 4,194,904 to Foerster et al.
In the Foerster process the metal components (grids, plates and the like) are first oxidized to yield a molten lead and a slag containing lead oxide and antimony oxide. The slag is then treated to remove the antimony by fuming. Being an oxidative process, this method is not applicable to recovery of lead from lead paste oxides and sulfates.
Treatment of the paste fraction presents particular problems in comparison with treatment of the elemental metals and alloys because of the presence of sulfates in the paste. U.S. Pat. No. 4,571,261 to Buren et al., describes a process in which the batteries are first crushed and separated into constituent fractions. The battery paste fraction is preferably removed by washing and screening from the plates, grids and case material. The non-metallic lead components (primarily lead oxides and sulfates) are then reduced in the absence of a slag or flux at 1050.degree. C. to 1250.degree. C. so as to obtain a molten lead phase in which the alloying elements such as antimony are dissolved. That process suffers from the disadvantage that the resultant lead has about 1.2% to 3.0% of antimony which must subsequently be removed and secondly, it is a relatively energy-intensive and costly process to conduct. This is because the process must be conducted at temperature above 1050.degree. C. and because the battery paste must be dried prior to use.